The recent widespread availability of tunable lasers has enhanced the interest in photochemical processes. The output of a carbon dioxide (CO.sub.2) infrared laser is resonant with the vibrational frequencies of a wide range of organic molecules, and because of this resonance, this type of gas laser has become the most popular for studies of infrared laser-induced chemical processes. The absorption of the laser radiation by the molecules promotes the molecules into excited vibrational states, and the molecules can become very reactive as a result.
In principle, the energy of the laser can be deposited into a single vibrational mode and the vibration can be excited to the point of dissociation. The resulting reactive species would be expected to react further. However, energy relaxation within a given vibrational mode generally occurs on a time scale of picoseconds. For complex molecules there is also a redistribution of energy among different vibrational modes and rotational and translational levels. Furthermore, at pressures of a few torr and higher, intermolecular redistribution accompanies collisions. Consequently it has been concluded that only a few microseconds are required for a molecule (which has been excited by an infrared laser) to reach a thermal equilibrium. Once the energy of the laser has been distributed throughout the molecule, any reaction which proceeds would very likely be similar to an ordinary pyrolysis reaction. Even if the reactions are governed by thermal processes, the laser-induced reactions will generally differ from ordinary pyrolytic thermal reactions because wall reactions are essentially eliminated from the former. In that case, it should be possible to compare the results of laser-induced "thermal" reactions with those carried out in shock tubes.
As noted hereinabove laser energy has been deposited to achieve a level of excitation to the point of dissociation of molecules to give reactive species. In experiments the laser energy has been distributed throughout the molecules to proceed along a reaction mode which is somewhat similar to an ordinary pyrolysis reaction. The tunable CO.sub.2 lasers have also been employed in elimination reactions. In fact, elimination reactions have been extensively investigated in polyatomic molecules exposed to an intense infrared laser field. The results obtained with organic halides have been the elimination of hydrogen halide and the formation of an alkene or alkyne compound. Some fragmentation of the parent molecule is also observed depending on the molecular size and laser fluence. However, recombination of radicals or thermally excited parent molecules to form larger molecules is usually not observed.
Pyrolysis of organic halides is found to follow several reaction paths, including unimolecular, radical chain, and bimolecular reactions. These reactions can produce simple and complex molecules by decomposition and radical addition.
As evidenced by the experimental activities relating to the use of tunable CO.sub.2 lasers, a technology tool is now available which can lead to discoveries formerly associated only with thermal chemistry which produced many products in addition to the product desired thereby increasing costs associated with separation and purification of the desired product.
An object of this invention is to provide a laser photochemical synthesis method for the production of benzene and substituted benzenes.
A further object of this invention is to provide a laser photochemical synthesis method for the production of benzene and substituted benzenes wherein the yield of benzene and substituted benzenes are pressure--and laser power--dependent.
Still a further object of this invention is to provide a laser photochemical synthesis method wherein the final product produced is controlled by the allyl halide selected for laser-induced reaction and the predetermined pressure and laser power for the laser-induced reaction for production of benzene or substituted benzenes.